This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Photonic integrated circuits based on high-index-contrast waveguides such as silicon, silicon-nitride, or InP have great potential in optical communications systems because of, e.g. potential reduction of size, power, and cost of such devices relative to solutions using lower index-contrast waveguide. However, one challenge associated with using such circuits in production wavelength-division multiplexed (WDM) systems is the wavelength accuracy of the fabricated circuits, e.g., whether the wavelength channels of a multiplexer and demultiplexer are aligned with the laser wavelengths of the multiplexed channels, e.g. the grid of the WDM system. For example, for a WDM system with 100 GHz spacing, the multiplexer/demultiplexer (mux/demux) channels typically need to be aligned to the grids with an accuracy of about +/−10 GHz. On the contrary, a mux/demux based on thin silicon waveguides typically have a wavelength variation of as large as 1000 GHz even on the same wafer, e.g. orders of magnitude higher than the desired grid alignment accuracy. Unfortunately, this variation is thought to be fundamental to currently known fabrication methods. For example, to achieve 10 GHz wavelength accuracy on thin silicon waveguides, the precision of the waveguide dimension (e.g. width) may need to be as small as 1 nm over the length of the waveguide, which is beyond the best known manufacturing capability. Although thermo-optical tuning can be used to shift the mux/demux channel positions, the magnitude of the possible adjustment is typically limited. For example, to obtain 1000 GHz tuning range of a thin-waveguide mux/demux, the waveguide temperature may need to be changed by as much as 100° C. for silicon waveguides, and as much as several hundred ° C. for devices based on silicon nitride. Such large changes are impractical due to, e.g. power consumption and resulting device reliability issues.